Currently, brine is electrolyzed to produce hydroxide and chlorine by employing a so-called ion exchange membrane method (refer to the below formula (1)). While its theoretical decomposition voltage is about 2.25 V, the operation is practically conducted at about 3 V due to the ohmic potential drop and the overpotential of an electrode existing in the system.2NaCl+2H2O→Cl2+2NaOH+H2  (1)
The chloroalkali industry consumes a great deal of energy. Accordingly, for significant energy saving, a method is investigated which includes a reaction in which a gas diffusion electrode is used as a cathode to reduce oxygen (refer to the below equation (2), and the reaction will be hereinafter referred to as “oxygen cathode method”).2NaCl+½O2+H2O→Cl2+2NaOH  (2)
This method lowers the theoretical decomposition voltage to 1.14V. Due to the ohmic loss and the electrode overvoltage, the practical operation is conducted at about 2 V. Since no hydrogen is generated, the energy saving of 30% or more can be expected.
As one of the oxygen cathode methods, a method is proposed in Japanese patent laid open gazette No. 11-124698 in which the gas diffusion electrode is in close contact with the ion exchange membrane to practically eliminate the cathode liquid chamber or in which the cathode chamber is configured as a cathode gas chamber. This method is referred to as a two-chamber method because the electrolytic cell consists of the anode chamber and the cathode gas chamber. This method has an advantage that the electrolysis voltage can be reduced to minimum because the anode, the ion exchange membrane and the cathode are in contact with one another to reduce the interelectrode resistance to the minimum.
In order to hold the electrolyte (catholyte) uniformly on the entire surface by closely contacting the gas diffusion electrode on the ion exchange membrane in this method, an elastic material (cushion material) is elastically accommodated in the cathode chamber so as to press the gas diffusion electrode to the anode through the ion exchange membrane by using the repulsive force generated therein. In order to hold the electrolyte more securely, a carbon cloth having good fluid retaining ability may be sandwiched between the ion exchange membrane and the gas diffusion electrode (Japanese patent gazette No. 3553775). Use of a mat or a coil prepared by stacking demister meshes as the cushion material is under consideration. The mat is obtained by stacking a plurality of metal wires which are subjected to stockinette stitch and a wave making process. The depth of the waves is about 2 to 10 mm. The wave making process generates a repulsive force. On the other hand, the coil is obtained by roller finish. The coil axis is disposed parallel to the back plate of the cathode gas chamber. The repulsive force is generated when the coil ring is compressed along its diameter. The coil diameter is 2 to 10 mm.
High concentration oxygen, water vapor and caustic soda mist which makes a severe corrosion environment exist in the cathode gas chamber of which a temperature reaches around 90° C. so that the cushion material is required to be excellently corrosion-resistive. The cushion material also has a role of discharging current from the gas diffusion electrode to the back plate of the cathode gas chamber. The cushion material is made of nickel or high nickel alloy which satisfies the above requirements.
Oxygen is supplied from the rear of the gas diffusion electrode to the inside thereof in the cathode gas chamber. Accordingly, the thinner cathode gas chamber is advantageous. On the other hand, in the electrolytic cell having several square meters of the active area, the thicknesses of the cathode gas chamber spread in several millimeters depending on their positions, and the compression displacements of the cushion material differ from one another in several millimeters depending on their positions resulting in the generation of the difference of the repulsive forces exerting on the gas diffusion electrode. In order to control the repulsive force in the required and accepted range, the average thickness of the cathode gas chamber is established from 4 to 10 mm.
The repulsive force is generally recognized as follows.
A liquid pressure of brine is exerted in the anode chamber and a gas pressure is exerted in the cathode gas chamber which is separated from the anode chamber by an ion exchange membrane. The typical depth of the brine in the anode chamber is about one meter, and the pressure at the deepest part is about 11 kPa. On the other hand, the cathode gas chamber pressure at the uppermost part of the inlet is only about 1 to 2 kPa. The cushion material is required to supply the repulsive force sufficient to compensate for the above pressure difference. An insufficient repulsive force separates the anode from the ion exchange membrane and the entire gas diffusion electrode, thereby elevating the voltage. The repulsive force is generally established between about 12 to 20 kPa.